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Coral reefs are the most diverse of marine ecosystems, with hundreds of thousands if not millions of species associated with reefs. For this reason they are often called the rain forests of the sea, and as such they have been of interest not only to marine scientists but also to ecologists and evolutionary biologists broadly. The center of reef diversity is the Indo-West Pacific, especially the “coral triangle,” comprised of the island-rich waters of Southeast Asia (e.g., Indonesia and the Philippines). The next most extensive area of reef building and reef diversity is in the tropical...

Coral reefs are the most diverse of marine ecosystems, with hundreds of thousands if not millions of species associated with reefs. For this reason they are often called the rain forests of the sea, and as such they have been of interest not only to marine scientists but also to ecologists and evolutionary biologists broadly. The center of reef diversity is the Indo-West Pacific, especially the “coral triangle,” comprised of the island-rich waters of Southeast Asia (e.g., Indonesia and the Philippines). The next most extensive area of reef building and reef diversity is in the tropical western Atlantic. In the early 21st century most reefs are built primarily by stony corals in the order Scleractinia (phylum Cnidaria, class Anthozoa), but large stony structures have in the past been built by microbes, clams, and other organisms. Coral reefs are dynamic structures that are constantly being both built by the skeletons of corals and other organisms, on the one hand, and broken down into rubble and sand, on the other. The ability of corals to outpace the destructive forces of storms and bioeroders is due to their symbiosis with single-celled algal symbionts in their tissues. These zooxanthellae (a diverse group of dinoflagellates once thought to be a single species) transfer some of the products of their photosynthesis to their coral hosts, which in return provide the zooxanthellae with nutrients. Also like rain forests, coral reef ecosystems are highly threatened around the world, and many have been substantially affected by humans for centuries. Local impacts, such as habitat destruction, overfishing, pollution, and invasive species, have been joined by the global impacts of ocean warming and acidification. The result has been that many coral reefs have been taken over by macroalgae or other space occupiers that grow faster than corals and are less sensitive to stressors associated with human activities. Coral disease, which is increasing due to warming and pollution, and coral bleaching, caused primarily by ocean warming, are now important causes of coral mortality. Overall, about one-half of all living coral may already have been lost, and one-third of all coral species are now deemed to be at risk of extinction; this places them on par with the most endangered terrestrial organisms. Because carbon dioxide emissions are continuing to rise, the best hopes in the short term for conserving and restoring coral reefs involve reducing local stressors. Measures to improve water quality and reduce overfishing (particularly of herbivores) are critical components of coral reef management strategies. However, reducing carbon dioxide emissions will be essential for the long-term survival of coral reefs.